As communications technologies become increasingly mature, communication speeds are increasingly high, and people are beginning to pay increasing attention to communication security. Quantum cryptography communication is a new communications technology combining quantum properties and conventional cryptography. Security in a communication transmission process is ensured by basic principles and properties of quantum mechanics. Developing for more than three decades, Quantum cryptography communication has now begun to find practical usage on the market.
Quantum cryptography communication is mainly used for key distribution, namely, quantum key distribution (Quantum Key Distribution, QKD). A QKD system is configured to generate and distribute a quantum key. This quantum key is used to encrypt classical information, enhancing security in a transmission process of classical information. A one-way QKD system is used as an example. An operating principle of the system is: A sender side randomly encodes a string of information for a quantum optical pulse signal in a quantum state; the information is detected by a detector of a receiver side after being transmitted through a quantum channel; and then the sender side and the receiver side run classical channel procedures such as data comparison, so that the sender side and the receiver side finally share a same group of secure random number keys.
Currently, there are two QKD manners: discrete-variable quantum key distribution (Discrete-Variable Quantum Key Distribution, DV-QKD) and continuous-variable quantum key distribution (Continuous-Variable Quantum Key Distribution, CV-QKD). Being discrete and continuous, it is indicated whether the randomly encoding information in a quantum state is discrete or continuous.
In DV-QKD, key distribution is implemented by encoding a single-photon signal, and therefore high detection accuracy and a single-photon detector that works in a low-temperature environment are required. However, these are not required in CV-QKD, and a balanced homodyne detector is used instead. Therefore, CV-QKD is more practicable. Moreover, properties of CV-QKD make CV-QKD well compatible with a current wavelength division multiplexing network.
FIG. 1 shows a conventional CV-QKD system. As shown in FIG. 1, in a conventional CV-QKD system solution, a continuous laser light source of a sender side (referred to as Alice) generates a periodic optical pulse signal through chopping by an amplitude modulator. A 1:9 beam glitter splits the optical pulse signal into two signals, and a pulse signal with a higher light intensity is directly input into one end of a polarization coupler as an associated local-frequency optical pulse signal. For the other pulse with a lower light intensity, random parameter modulation continues to be performed by using an amplitude and phase modulator. By loading a corresponding voltage on the modulator, a to-be-sent random number may be encoded onto a regular component of the signal with a lower light intensity. A faraday rotation mirror in this modulation line is configured to change polarization of the pulse signal so that the signal rotates by 90° on an original polarization basis (this means that polarization of the optical pulse signal is perpendicular to polarization of the local-frequency optical pulse signal). In addition, the optical pulse signal goes for another length of optical path, and is then attenuated by an attenuator to become a quantum optical pulse signal that is input into another end of the polarization coupler. In this way, this quantum optical pulse signal can be transmitted together with the local-frequency optical pulse signal in the other line in an optical fiber through polarization-division multiplexing and time-division multiplexing. Because an intensity of the local-frequency light is much higher than that of the quantum optical pulse signal, the local-frequency light tends to cause crosstalk to the quantum optical pulse signal. Therefore, the polarization-division multiplexing and time-division multiplexing are used to increase isolation between the local-frequency light and the signal light.
At the receiver side (usually referred to as Bob), a polarization state of the input signal is controlled and adjusted in real time by using a dynamic polarization controller, so that when the input signal passes through a subsequent polarization optical splitter, the quantum optical pulse signal is completely output from one end, and the associated local-frequency optical pulse signal is completely output from another end. The associated local-frequency optical pulse signal needs to pass through a phase modulator for random selection of a measurement base, and pass through an unequal arm optical path apparatus the same as that of the sender side for time delay compensation for the local-frequency optical pulse signal, thereby ensuring that the quantum optical pulse signal and the local-frequency light that are input into a 2×2 coupler are aligned in timing. Two output ends of the 2×2 coupler are connected to two input ends of a balanced receiver. After data of an electrical signal output by using the balanced receiver is collected, initial data obtained in some data processing methods is usually referred to as a raw key (raw key).
In the conventional CV-QKD system solution, an associated local-frequency light manner is used, to be specific, when a quantum optical pulse signal is transmitted, a classical optical pulse signal of a higher light intensity is also transmitted as a local-frequency optical pulse signal. Because this local-frequency optical pulse signal interferes greatly with the quantum optical pulse signal, to minimize interference, a manner in which a quantum optical pulse signal and a local-frequency optical pulse signal are time-division multiplexed is usually used in practice. To be specific, a pulse of the quantum optical pulse signal pulse is delayed for a length of time at the sender side, so that the quantum optical pulse signal and the associated local-frequency optical pulse signal are transmitted at different times. However, this requires that a delay compensation of the same length of time be applied to the local-frequency light at the receiver side. Therefore, optical fibers of a same length for delay adjustment heed to be made for both the receiver side and the sender side, which is of great difficulty. In addition, the local-frequency optical pulse signal is related to a measurement level of vacuum noise of a balanced detector at the receiver side, and therefore an intensity change of the local-frequency optical pulse signal may affect a detection result. If the change is implemented by a third-party eavesdropper, system security may be affected (a local-frequency optical pulse signal attack).
A self-referenced continuous-variable quantum key distribution (SR-CV-QKD) solution is proposed in March, 2015 by both Bing Qi et al. in “Generating the local oscillator “locally” in continuous-variable quantum key distribution based on coherent detection” and Daniel B. S. Soh et al. in “Self referenced continuous-variable quantum key distribution”. Local-frequency light generation is moved from a sender side as in a conventional CV-QKD system to a receiver side, and to ensure that an interference result between a local-frequency optical pulse signal generated at the receiver side and a quantum optical pulse signal light sent by the sender side can be parsed, a reference pulse with a higher light intensity is introduced between quantum optical pulse signals sent by the sender side. This reference pulse may be referred to as reference light, used to detect a frequency difference and a phase difference between the quantum optical pulse signal light sent by the sender side and the local-frequency optical pulse signal of the receiver side. In this way, measuring machine selection is implemented in the CV-QKD system, and information in the quantum optical pulse signal is recovered. Using this solution does not require that a length of an optical fiber at the sender side and that at the receiver side be strictly controlled to be equal. In addition, the reference pulse has a much lower intensity than the local-frequency optical pulse signal sent by the sender side of the conventional CV-QKD system, causing less interference to the quantum optical pulse signal. Therefore, a hidden risk of a local-frequency optical pulse signal attack coming with the sending of the local-frequency optical pulse signal, is eliminated.
In the SR-CV-QKD system solution, estimation is performed through measurement of a phase difference between the local-frequency light and the reference light. This estimation is on a basis that the reference signal and the quantum signal light have extremely approximate pulses in a time domain, and that the two signals have both been transmitted on a same segment of a channel. Therefore, the estimation may be performed by approximately considering that a phase of the reference light, changing in the transmission process, is the same as that of a pulse of neighboring quantum signal light, or by using an average value of a pulse phase of the reference light before the quantum signal light and a pulse phase of the reference light after the quantum signal light. However, because the local-frequency optical pulse signal and the quantum optical pulse signal is not generated by one laser, a delay between the local-frequency optical pulse signal and the quantum optical pulse signal needs to be adjusted so that the local-frequency optical pulse signal and the quantum optical pulse signal are aligned in timing, ensuring optimum interference. Currently, timing alignment is implemented by using an optical delay line. However, a tiny fluctuation, if occurring in an optical transmission path, leads to a timing jitter of a signal pulse, resulting in an extra delay and unaligned timing between the two signals. To ensure timing alignment, the optical delay line needs to be adjusted continually. This results in relatively low timing alignment efficiency.